diff --git a/packages/quicktype-core/src/language/Java/utils.ts b/packages/quicktype-core/src/language/Java/utils.ts
index 8e27c72b6e..80c2cfc1b8 100644
--- a/packages/quicktype-core/src/language/Java/utils.ts
+++ b/packages/quicktype-core/src/language/Java/utils.ts
@@ -50,7 +50,9 @@ export function javaNameStyle(
         upperUnderscore || startWithUpper
             ? allUpperWordStyle
             : allLowerWordStyle,
-        acronymsStyle,
+        // For UPPER_UNDERSCORE style (Java enum constants), always use allUpperWordStyle for acronyms
+        // to maintain consistency with the naming convention (e.g., XXX_SPA_XXX)
+        upperUnderscore ? allUpperWordStyle : acronymsStyle,
         upperUnderscore ? "_" : "",
         isStartCharacter,
     );
diff --git a/test/check-java-acronym-names.ts b/test/check-java-acronym-names.ts
new file mode 100644
index 0000000000..9480553c45
--- /dev/null
+++ b/test/check-java-acronym-names.ts
@@ -0,0 +1,105 @@
+// Guard: Java enum constants must keep acronyms uppercase for every
+// --acronym-style setting.
+//
+// Java enum constants are rendered in UPPER_UNDERSCORE style. Before the fix
+// in https://github.com/glideapps/quicktype/pull/2851 (issue #2850), the
+// acronym-style option was still applied to words recognized as acronyms
+// (e.g. "SPA"), so with --acronym-style=camel the JSON enum value
+// "MULTI_SPA_IN_GROUP_REJECTED" produced the constant
+// "MULTI_Spa_IN_GROUP_REJECTED" (and "MULTI_spa_IN_GROUP_REJECTED" with
+// lowerCase).
+//
+// The regular fixture harness cannot catch this: the mangled constants are
+// self-consistent identifiers, so the generated code still compiles, and
+// (de)serialization uses the raw JSON names, so round-tripping succeeds.
+// This check instead generates Java code directly and asserts on the emitted
+// *identifier*. Note that we must NOT just check that the output contains
+// "MULTI_SPA_IN_GROUP_REJECTED" — the raw JSON name always appears in the
+// string literals of toValue()/forValue(), even when the identifier is
+// mangled. We extract the identifier from `case <ident>: return "<name>";`
+// and compare that.
+
+import { InputData, JSONSchemaInput, quicktype } from "quicktype-core";
+
+// "spa" is a known acronym (see Acronyms.const.ts), so acronym styling would
+// apply to the SPA word if the fix regressed.
+const enumValue = "MULTI_SPA_IN_GROUP_REJECTED";
+
+const schema = JSON.stringify({
+    $schema: "http://json-schema.org/draft-06/schema#",
+    type: "object",
+    properties: {
+        messageCode: {
+            type: "string",
+            enum: [enumValue],
+        },
+    },
+    required: ["messageCode"],
+});
+
+const acronymStyles = ["original", "pascal", "camel", "lowerCase"];
+
+async function javaEnumConstantIdentifier(
+    acronymStyle: string,
+): Promise<string> {
+    const schemaInput = new JSONSchemaInput(undefined);
+    await schemaInput.addSource({ name: "TopLevel", schema });
+    const inputData = new InputData();
+    inputData.addInput(schemaInput);
+
+    const result = await quicktype({
+        inputData,
+        lang: "java",
+        rendererOptions: { "acronym-style": acronymStyle },
+    });
+    const output = result.lines.join("\n");
+
+    // Matches e.g. `case MULTI_SPA_IN_GROUP_REJECTED: return "MULTI_SPA_IN_GROUP_REJECTED";`
+    // in the generated MessageCode.toValue() — the capture is the identifier.
+    const match = output.match(
+        new RegExp(`case (\\w+): return "${enumValue}";`),
+    );
+    if (match === null) {
+        console.error(
+            `error: could not find the enum constant for "${enumValue}" in the generated Java code (acronym-style=${acronymStyle}):\n\n${output}`,
+        );
+        process.exit(1);
+    }
+
+    return match[1];
+}
+
+export async function checkJavaEnumAcronymCasing(): Promise<void> {
+    const failures: string[] = [];
+    for (const style of acronymStyles) {
+        const identifier = await javaEnumConstantIdentifier(style);
+        if (identifier !== enumValue) {
+            failures.push(
+                `    acronym-style=${style}: got "${identifier}", expected "${enumValue}"`,
+            );
+        }
+    }
+
+    if (failures.length > 0) {
+        console.error(
+            `error: Java enum constants must keep acronyms uppercase for every acronym-style (issue #2850):
+
+${failures.join("\n")}
+
+javaNameStyle must force allUpperWordStyle for acronyms in UPPER_UNDERSCORE
+names — see packages/quicktype-core/src/language/Java/utils.ts and
+https://github.com/glideapps/quicktype/pull/2851`,
+        );
+        process.exit(1);
+    }
+}
+
+// Allow running the check standalone:
+//   NODE_PATH=`pwd`/node_modules npx ts-node --project test/tsconfig.json test/check-java-acronym-names.ts
+if (require.main === module) {
+    checkJavaEnumAcronymCasing().then(() => {
+        console.error(
+            "* Java enum constants keep acronyms uppercase for every acronym-style",
+        );
+    });
+}
diff --git a/test/test.ts b/test/test.ts
index f9f4e316a0..5bd0b245c1 100755
--- a/test/test.ts
+++ b/test/test.ts
@@ -5,6 +5,7 @@ import { inParallel } from "./lib/multicore";
 import { execAsync, type Sample } from "./utils";
 import { type Fixture, allFixtures } from "./fixtures";
 import { affectedFixtures, divideParallelJobs } from "./buildkite";
+import { checkJavaEnumAcronymCasing } from "./check-java-acronym-names";
 import { checkCoreHasNoNodePrefixedImports } from "./check-no-node-imports";
 
 const exit = require("exit");
@@ -21,6 +22,11 @@ async function main(sources: string[]) {
     // use "node:"-prefixed imports or it breaks web bundlers (issue #2763).
     checkCoreHasNoNodePrefixedImports();
 
+    // Regression check for issue #2850: Java enum constants must keep
+    // acronyms uppercase for every --acronym-style. The fixture harness
+    // can't catch this (mangled constants still compile and round-trip).
+    await checkJavaEnumAcronymCasing();
+
     let fixtures = affectedFixtures();
     const fixturesFromCmdline = process.env.FIXTURE;
     if (fixturesFromCmdline) {